1. Field of the Invention
The generation of ever smaller structures which can be read out again in a reproducible manner is of considerable interest, in particular when used for data storage. The storage density of such media is determined by the smallest structure which can be written and read reproducibly. The achievable dimensions of the generated structures are limited at the lower end in two respects. Firstly, the positional accuracy and the local resolution limits the achievable storage density for example due to the geometry of the read/write head. Secondly, however, material properties, for example material inhomogeneities or the size of the Wei.beta. domains, do not allow the size of generated structures to be reduced as desired. The lower limit for writing smaller and smaller information units into a solid is set in principle by the distance between adjacent atoms. An object which is desirable for high storage densities is to come very close to this fundamental limit, i.e. to be able to store the 1 bit information unit on a dimension of only a few atomic diameters.
2. Description of the Related Art
Near field methods using local probes, for example the scanning tunneling microscope (STM) or the scanning force microscope in the form of an atomic force microscope or magnetic force microscope, are the obvious choice for such storage processes, not only due to their high positional accuracy of better than 0.1 nm but in particular also due to their high local resolution. Thus, it has already proved possible to "write" letters on nickel using xenon atoms at 4K and in an ultra-high vacuum by specifically positioning xenon atoms using the tip of an STM (D. M. Eigler and E. K. Schweitzer, Nature 344 (1990) 524).
However, the possibility of writing and reading under ambient conditions, i.e. at room temperature and in air, is also crucial for the industrial application of extremely small structures of this type in addition to their dimensions and the reproducibility of the writing process. The structures produced should also prove to be time-stable under these conditions.
The production of structures of this type which are time-stable at room temperature is possible, for example, by mechanically inserting the tip into the sample (van Loenen et al., Appl. Phys. Lett. 55 (1989) 1312). At the same time, however, the surface of the sample is in most cases destroyed locally due to bond breaking. This in turn has the disadvantage that the process is no longer reversible and thus subsequent erasure or modification of the information is no longer possible once it has been written. Rather, it is desired to have a process which allows small deformations having dimensions in the nanometer range to be produced on the surface of a solid without destruction of the atomic order of the surface, since modifications of this type include the potential for erasability, for example by thermal means.